Ali Akbar Velayati, Iran's Supreme Leader Ayatollah Ali Khamenei's top adviser on international affairs, smiles as he listens to questions from the media during a news conference after meeting with Lebanon's Prime Minister Tammam Salam at the government palace in Beirut May 18, 2015. REUTERS/Mohamed Azakir/File Photo

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Ali Akbar Velayati, Iran's Supreme Leader Ayatollah Ali Khamenei's top adviser on international affairs, smiles as he listens to questions from the media during a news conference after meeting with Lebanon's Prime Minister Tammam Salam at the government palace in Beirut May 18, 2015. REUTERS/Mohamed Azakir/File Photo

Beirut: A top adviser to Iran's Supreme Leader Ayatollah Ali Khamenei hailed his country's "strategic relationship" with Russia on Wednesday during a visit to Moscow, part of a diplomatic offensive by Tehran as it braces for renewed US sanctions.

Ali Akbar Velayati, who is due to meet President Vladimir Putin, also took a swipe at US President Donald Trump, saying his "unreliable" actions made Tehran's close ties with Moscow all the more necessary.

The United States pulled out of a multinational deal in May to lift sanctions against Iran in return for curbs to its nuclear programme.

Washington says it will reimpose sanctions and has told other countries to halt all imports of Iranian oil from November 4 or face US financial measures.

"The relationship between Iran and the Russian Federation is a strategic relationship and in recent years the collective bilaterial and regional relations have expanded," Iran's ISNA news agency quoted Velayati as saying on arrival in Moscow.

He said he would deliver messages to Putin from Iran's Supreme Leader and President Hassan Rouhani and that they would discuss the "very sensitive" global situation.

Velayati cited Russian and Iranian cooperation in Syria, where they both support President Bashar Al Assad's forces in the seven-year Syrian civil war against what the Iranian official called "terrorism and its sponsors".

"Only a strategic and long-term relationship (with Russia) can continue this cooperation," said Velayati, who was also due to meet Russian Energy Minister Alexander Novak in Moscow.

Velayati, who is also expected to visit China in the near future, described Trump as an "unreliable individual... with relation to international law who necessitates more and more cooperation (between Iran and Russia)".

Hossein Amir-Abdollahian, a special adviser on international affairs to the speaker of Iran's parliament, said Velayati was scheduled to meet Putin on Thursday and would discuss ways to confront US policy in the Middle East region.

"A frank dialogue with Russian leaders at the highest levels and exchanging views can lead to the views of the two countries moving closer and help in finding ways to increase stability and security in the region and confront the improper policies of America and its allies," Amir-Abdollahian, a former deputy foreign minister, said on Wednesday, according to state media.

A US State Department official said this month that Washington's goal was to get as many countries as possible down to zero Iranian oil imports. However, Secretary of State Mike Pompeo said on Tuesday the United States might consider requests from some countries to be exempted from sanctions.

Apart from the United States, the other signatories to the 2015 Iran nuclear deal - Russia, China, Germany, France and Britain - have said they still support the accord.

But European firms are reluctant to risk far-reaching US financial penalties they would face for doing business in Iran.